Assumptions in Decision Making: How to Know What You Don’t Know

Most professionals don’t think of themselves as making assumptions. They think they’re working from experience, pattern recognition, or common sense. The problem is that assumptions and verified facts occupy the same mental space and feel identical from the inside.

Hidden assumptions shape decisions. When beliefs are treated as facts without verification, even well-reasoned plans can fail in execution. Structured techniques like pre-mortems and assumption mapping helps to make hidden assumptions visible. The goal is not to eliminate assumptions (every decision involves them) but to distinguish the ones that have been examined from the ones that haven’t.

The further into execution a plan gets, the more expensive and time-consuming an unchecked assumption becomes. Surface one in a planning meeting and it’s a ten-minute conversation. Raise the same assumption after a rollout and it can mean weeks of rework, missed deadlines, or damaged relationships with stakeholders. Strong critical thinking skills are what make the difference between catching that assumption early and discovering it late.

Key Insights

• Assumptions feel like knowledge, which is why they don’t get questioned. The brain doesn’t flag familiar beliefs the way it flags unfamiliar information.
• The cost of an unchecked assumption compounds with time. The same assumption that takes two minutes to bring up in planning can take weeks to untangle in execution.
• Standard critiquing methods are less effective at exposing assumptions than techniques that force a deliberate change in perspective.
• Teams that build assumption-checking into their process make faster, more confident decisions because they’re not backtracking.
• PMC facilitators consistently observe that the assumptions most likely to derail a plan are often the ones the team agreed on fastest.

What Is an Assumption in Decision Making?

An assumption in decision making is any belief that is treated as true without being explicitly verified. In professional settings, assumptions generally fall into three categories.

Working assumptions

A project manager assumes that a vendor relationship will continue for another quarter. A team lead assumes that meeting rooms will be available for a planning session. These are low-stakes, low-consequence beliefs that may be impractical to verify every time.

Unexamined assumptions

A team scopes a project around a budget that was discussed once but never formally confirmed. A communication plan is built on the assumption that everyone reads email daily. A timeline is drafted without accounting for the fact that two key contributors will be on leave during a critical phase. Nobody flagged these as assumptions because they felt like facts.

False certainties

A stakeholder is assumed to be aligned because they didn’t push back in the last meeting. A process is assumed to be working because it worked the last time. A decision is assumed to be low-risk because the risk was never explicitly examined.

These are assumptions that the team has collectively stopped questioning.

One reason all three types can be hard to distinguish is that the brain isn’t designed to flag its own gaps. The Change Cycle framework, which is the foundation of PMC’s Coping with Change course, captures this pattern directly: when people are missing information, the brain tends to fill those gaps automatically, either by making assumptions or by distorting available facts to fit an existing picture.

This means that assumption-checking has to be deliberate, because the brain won’t do it on its own.

Why Assumptions Are So Hard to Catch

The reason experienced professionals miss assumptions is that the mental processes that create them operate quickly and largely outside conscious awareness.

A peer-reviewed study published in Frontiers in Psychology examined cognitive biases across professional decision making in management, finance, medicine, and law. One of its central findings was that competent, well-intentioned professionals can be systematically biased without knowing it. Knowing a bias exists and being immune to it are two different things.

For assumptions specifically, this plays out in a predictable way. The more experienced a professional is, the faster their pattern recognition operates. Speed is an asset in most situations. In assumption-checking, it’s a liability. An experienced manager who has run ten similar projects will move through the planning phase quickly, drawing on what worked before. The assumptions embedded in that prior experience travel with them, unexamined, because nothing has triggered a reason to question them.

This is also why group settings don’t automatically solve the problem. When a team reaches consensus quickly, it can feel like the plan is solid. In practice, fast agreement sometimes means the group has converged on a shared assumption rather than a verified fact. This is one of the patterns PMC facilitators see most consistently in critical thinking and project management courses: the assumptions most likely to derail a plan are often the ones the team agreed on fastest.

There’s a related issue that shows up in how plans get reviewed. Most teams critique a plan by asking “what could go wrong?” That question is useful, but it defaults to surface-level risks. It rarely forces the team to examine the beliefs the plan is actually built on.

What Unchecked Assumptions Cost

The cost of an unchecked assumption typically becomes visible once a plan is underway, and by then the options for addressing it have narrowed.

Consider a few scenarios that PMC facilitators have encountered in workshop settings:

• A project is weeks into execution when it comes to light that a key dependency on another team member was never confirmed. It was assumed that they had capacity and had been informed. Neither was true. Now the team is choosing between waiting for that person to become available, bringing in someone else who lacks the context and history the project needs, or moving forward short-handed and accepting the gaps that come with it.
• A department is offered the opportunity to move to a newly available floor. The team lead confirms the move without looping in IT, assuming it’s the same setup. It isn’t. The floor’s network infrastructure hasn’t been updated to handle a larger team’s bandwidth needs, meaning video calls drop or slow to a crawl during peak hours. By the time those issues emerge, the move is already underway and course-correcting is significantly more expensive and disruptive than it would have been at the planning stage.
• A stakeholder communication plan is built on the assumption that a key decision maker is aligned. That person was consulted while travelling and wasn’t able to give it their full attention, which led to a misunderstanding. They were on the call, so it was assumed they fully understood the plan. They didn’t. And they weren’t happy with the rollout.

The teams involved were capable and well-intentioned. What they were missing was a structured moment in the planning process where someone asked: what are we treating as true that we haven’t actually confirmed?

The quality of an outcome is directly shaped by the quality of the decision behind it. If the assumptions built into that decision were never examined, the problems they create don’t show up in the planning. They show up in the result.

Three Techniques to Reveal Hidden Assumptions

Each of the following techniques can be applied in a working session, a project kickoff, a planning review, or any meeting where decisions are being made.

The Pre-Mortem

The pre-mortem was developed by cognitive psychologist Gary Klein and first described in a Harvard Business Review article in 2007. The method works by asking a team to imagine, before a plan is executed, that it has already failed.

Standard critiquing asks “what might go wrong?” The pre-mortem asks “what did go wrong?” The shift from possibility to certainty changes the way the brain engages with the question. Assumptions that would have gone unvoiced in a normal review surface because the frame has been changed.

In practice, the pre-mortem takes 20 to 30 minutes. Each team member independently generates reasons the plan failed, then shares one at a time until all reasons are on the table. The facilitator records them without debate. The team then reviews the list to identify assumptions that can be verified or addressed before execution begins.

Research by Veinott, Klein, and Wiggins (2010), involving 178 participants evaluating a crisis management plan. Participants who used a pre-mortem showed a larger reduction in confidence in the plan than those using other review approaches, including standard critique and pros-and-cons style analysis.

After participants generated solutions to the problems they had identified, confidence increased again. The pre-mortem group showed the strongest recovery at that stage.

Assumption Mapping: The Importance/Certainty Matrix

Assumption mapping is a structured method for making hidden beliefs visible and then prioritizing which ones need attention. The approach used here draws on the Strategic Assumptions Surfacing and Testing framework developed at the Institute for Manufacturing at the University of Cambridge.

The process begins with a simple prompt: write down the current problem statement or plan objective, then list every assumption embedded in it. What must be true for this plan to work?

The team lists every assumption connected to resources, timelines, stakeholders, dependencies, or context.

Once the list exists, each assumption is placed on a matrix with two dimensions:

• Importance: If this assumption turns out to be wrong, how significantly does it affect the plan?
• Certainty: How confident is the team that this assumption is actually true?

The assumptions that land in quadrant one (high importance, low certainty) are the ones that need immediate attention. The plan depends on them, but the team cannot confirm them. Quadrant two (high importance, high certainty) becomes the plan’s working foundation.

Assumptions in the lower half can be deprioritized or monitored. The matrix doesn’t eliminate assumptions — It converts an invisible list of beliefs into a visible, prioritized set of items that can be checked, tested, or flagged as known risks.

The “What Must Be True” Question

The third assumption-busting technique is the simplest and the most portable. Before committing to a plan or a recommendation, the team asks one question: what must be true for this to work?

This reverses the usual direction of thinking. Most planning moves forward: here is what we will do, and here is why. “What must be true” moves backward: here is where we want to end up, so what has to be true to get there?

The answers help to reveal assumptions that weren’t visible in the forward-planning pass. A plan that assumes a specific budget, a particular vendor, a certain level of stakeholder support, or a timeline free of competing priorities will show all of those dependencies when the team works through what must be true for the plan to succeed.

This question can be used in a five-minute check at the end of a planning conversation or as a structured exercise where each assumption generated is written down and assigned to someone for verification. Consistency matters ore than format.

How to Make Assumption-Checking a Team Habit

Individual awareness of assumption risk is a start. Team-level practice is what changes outcomes.

The shift from individual to team practice requires two things.

A shared vocabulary

When a team has a common language for distinguishing assumptions from facts, and a shared norm around identifying them, the conversation becomes faster and less threatening. Flagging an assumption stops feeling like a challenge to the person who proposed the plan and starts feeling like a contribution to making the plan stronger.

A designated moment

Assumption-checking works best when it is built into an existing process rather than added as a separate step. A pre-mortem at the project kickoff. A “what must be true” pass at the end of a planning session. An assumption column in the project log that gets reviewed at key milestones. These are lightweight interventions that take minutes but prevent the kinds of downstream failures that take weeks to recover from.

For managers and team leads, the most important contribution is modelling the behaviour. When a leader asks “what are we assuming here?” without it being read as criticism, it signals that identifying assumptions is valued, not penalized. That signal is what makes the practice stick.

The most in-demand skills in Canada’s workforce consistently include critical thinking and analytical reasoning. Assumption-checking is a skill, and like any skill, it develops with structured practice.

Take the Next Step in Your Decision Making

PMC Training’s Critical Thinking and Problem Solving for Effective Decision-Making course gives professionals hands-on practice with the tools covered in this article and more, including how to analyze context, evaluate information, and avoid the most common assumptions in decision making that derail otherwise sound plans. If your team is ready to build this into how they work, this is where that practice begins.